As discussed in detail in the above-incorporated U.S. Patents, the electrical resistance of a properly formed magnetic tunnel junction memory cell depends on a magnetic writing stimulus applied to the junction. The cell response is hysteretic and the cell therefore retains some memory of the applied magnetic writing stimulus. The remanent magnetic configuration of the tunnel junction device, and its resultant electrical resistance value, is the basis for the application of such devices to electrically accessed MRAM arrays.
To fabricate a large and reliably accessed MRAM array containing thousands or millions of cells on a single chip, uniformity and predictability of the magnetic response characteristic of each cell is of great importance. However, due to many factors related to manufacturing uncertainties and intrinsic magnetic variability, cell to cell response variations can be very large. This magnetic response variability from cell to cell directly impacts the electrical and resultant magnetic writing stimulus needed to access each cell, and therefore prevents array-wide selectivity to occur using a preferred, fixed electrical and resultant magnetic writing stimulus value.
As an example, and with reference to FIGS. 1a-b, in an MRAM array, cells are positioned at the intersections of an exemplary rectangular grid of electrically conductive lines 1-6. The lines are arranged over a substrate and cross, thereby forming intersecting regions at which the cells are positioned, e.g., cell 9. As discussed further below, each cell normally comprises a free magnetic region 24 and a reference magnetic region 22. (The term reference region is used broadly herein to denote any type of region which, in cooperation with the free or changeable region, results in a detectable state of the device as a whole.) The ability of this type of cell to store electrically accessible data hinges on electron tunneling between these two regions, which in turn is dependent on the relative directions of magnetization of these two regions. Rotating the magnetization in the free region into one of two (possible more) selectable directions in a bi-stable manner results in binary state stored in the cell. If the cell is oriented with its magnetic easy-axis ("EA") horizontal then an electrical writing current flowing through a vertical line will apply an EA magnetic field, and a current flowing through a horizontal line will apply a hard-axis ("HA") magnetic field, to the cell.
In one implementation of MRAM cells, the writing of individual cells adheres to a concept referred to as the "asteroid" for switching. The switching threshold of a single free region depends on the combination of EA and HA magnetic fields applied thereto. This "Stoner-Wohlfarth" asteroid model, shown in FIG. 2a, illustrates these threshold values in the plane of applied EA and HA fields. Switching occurs when a combination of EA and HA fields at the cell results in a vector outside of the asteroid. Vectors inside the asteroid will not switch the cell from one of its current bi-stable states. This asteroid model also illustrates how the EA field needed to switch a device is reduced in the presence of an HA bias field. Selectively switching a single cell within the array is achieved by applying electrical currents through a selected pair of horizontal and vertical lines. These currents generate a combination of EA and HA fields only at the cell located at the intersection of these lines, theoretically switching the selected cell, but not the neighboring cells.
All the cells along the horizontal line will experience the same applied HA field. Similarly all the cells along the vertical line will experience the same applied EA field. However, only the cell at the intersection of these lines will experience the combination of both fields necessary for switching.
Problems arise when the thresholds of the asteroid vary from cell to cell, and from hysteresis loop to hysteresis loop in the same cell. This leads to a broadening of the asteroid into a band of threshold values as shown in FIG. 2b. Since the ability to selectively switch cells hinges on all cells except one along a line not being switched under a single applied HA or EA field, if this band of the asteroid broadens too much, then it is no longer possible to selectively write individual cells, with equivalent writing stimuli, since other non-selected cells along the lines will also switch.
FIG. 3 illustrates the variability in the magnetic response for 12 neighboring MRAM devices, actually measured by the instant inventors, taking 2 EA writing loops for each device, for different HA bias fields (Hh). Each plot shows the measured resistance change, in percent, versus the applied EA field, for each given HA bias field (He). (These plots also illustrate how the hysteretic response of the cells depends on the applied HA bias field. To provide the above-mentioned cell selectivity using intersecting lines, it is desirable to operate with some applied HA bias field on one line, and an applied EA field on the other, i.e., H.sub.Y1 and H.sub.X2 in FIG. 2. However, it is also desirable to retain some hysteresis, so that the cell remains in one of two bi-stable states when the applied EA and HA fields are removed.) This variability can be summarized on an "asteroid" plot in which the HA bias field is plotted versus the EA coercive field for these loops (FIG. 4). For this set of devices, there is so much scatter that there is no effective applied stimulus operating window for these cells, i.e., there is no set of applied EA and HA fields that would switch each cell if applied thereto together, and that would switch none of the cells if either is applied separately.
Whether using the above-discussed asteroid selection model, or any other selection model, a major challenge in the successful implementation of an MRAM array with effective cell selectivity is the fabrication of many memory cells with nearly identical electrical and magnetic properties. This is particularly difficult for magnetic devices since their response is sensitive not only to local defects but also to edge or surface roughness.